Sub-meter GNSS accuracy with standard precision solutions? – a look at the Allystar 1201

Being able to get centimeter level errors with RTK or PPK solutions is a very powerful technique, but it does involve a certain amount of complexity and lack of robustness in more challenging conditions. In many cases, users don’t need quite this much accuracy and are looking for a simpler, more robust solution. In a recent post, I demonstrated that a u-blox F9P receiver can achieve sub-meter accuracy with it’s standard precision solution for static measurements and even better accuracy for dynamic measurements. This is a good choice for users who are primarily interested in a RTK/PPK solution but would like the standard precision solution as a backup. It is a relatively expensive solution however for those looking for just a standard precision solution. The Allystar 1201 dual-frequency GNSS module recently caught my eye as a possible answer for a lower cost option. It advertises sub-meter CEP standard precision solution and is available for only $35 on an EVK board or $13.50 for just the bare module in quantities of 10.

It does not have an internal RTK engine or provide access to the raw observations like the u-blox F9P does. This means that it can not be used for anything other than standard precision solutions, but if that is not a requirement, then this could be quite an attractive alternative to the F9P.

Here are the GNSS signals received and the accuracy spec for the Tau1201

Specs from Allystar 1201 datasheet

For comparison, here is the equivalent specs for the u-blox F9P

Specs from u-blox F9P datasheet

As you can see, the Tau1201 claims <1 meter CEP while the F9P claims 1.0 meter CEP. CEP stands for Circular Error Probability and is defined as the radius of a circle centered on the true value that contains 50% of the actual GNSS measurements. In other words, one half of the measurements should have an accuracy better than or equal to the CEP. Real-world performance will vary depending on atmospheric conditions, GNSS antenna, multipath conditions, satellite visibility and geometry. Neither spec lists specific test conditions but generally the specs assume good visibility, low multipath, nominal atmospheric conditions, and a reasonably high quality antenna.

I was curious to see how these two receivers compared in a real-world test so I ordered a Tau1201 EVK board from Digikey and collected some data. I ran for 24 hours with both receivers connected through an antenna splitter to a Harxon GPS500 survey grade antenna on the roof of my house. I configured both receivers to use all available constellations and output the internal solution with NMEA GGA messages.

Here are the resulting ground track plots for both receivers, u-blox F9P on the left, and the Allystar Tau1201 on the right. They are plotted with RTKPLOT and have statistics enabled from the options menu. I also set the coordinate origin in the options menu to the precise location of the antenna in WGS84 coordinates.

Ground tracks for 24 hours for the F9P and Tau1201

Here are the positions plotted versus time.

Position vs time for 24 hours for the F9P and Tau1201

The statistics may be difficult to read in the above plots, so I’ve copied them below.

u-blox F9P plot statistics

Allystar Tau1201 plot statistics

The standard deviations (STD) include only the measurement noise. The root-mean-squares (RMS) include both measurement noise and bias, so are the more relevant statistic in this case. We can combine the East and North measurements using the square root of the sum of the squares to get the horizontal RMS values (sometimes called 2drms). This gives us a value of 0.38 meters for the F9P and a value of 0.96 for the Tau1201. It is not possible to calculate the exact CEP metrics directly from the RMS values but we can estimate them assuming gaussian distributions and circular distributions. Neither of these assumptions is entirely true, but the estimate should still be reasonably accurate. Using a conversion factor of 1.19 derived in this article from GPS World, we get estimated CEP values of 0.32 for the F9P and 0.81 for the Tau1201.

In this test, both receivers achieved their advertised spec for CEP. Obviously, though, the more expensive F9P receiver outperformed the lower cost Tau1201. Exact results will vary based on some of the factor mentioned earlier (visibility, multi-path, atmospheric conditions, etc) but I would expect the relative differences between the two receivers to be fairly consistent.

These are for static measurements. The CEP values for dynamic measurements would be noticeably smaller due to the averaging out of the multipath errors as I demonstrated in the post referenced above.

Another interesting difference between the two measurements is the position errors averaged over 24 hours. The F9P average errors were less than 10 cm in both horizontal axes while the Tau1201 was small in the east direction, but was relatively large in the north direction at 63 cm. It’s hard to say from one measurement how consistent this difference would be over multiple measurements, but if it did hold up, then averaging the measurements over time to reduce the errors would be much more effective with the F9P than it would be with the Tau1201

In order to focus on the performance of the receiver, I used a relatively expensive antenna for this experiment, at least compared to the cost of the Tau1201. Although I did not repeat the test with any lower cost antennas, I would not expect significantly different results with any antenna intended for dual-frequency RTK solutions such as those from Ardusimple or DataGNSS, provided they are used with the appropriate ground plane. Even lower cost antennas, as long as they are intended for dual-frequency use, might work as well, but would require more testing and would probably have a larger effect on the results. If anybody has run a similar experiment with very low cost antennas and would like to share their results, please leave a comment below.

So, to summarize, the Tau1201 receiver is not able to match the performance of the F9P, but it is significantly less expensive, does achieve sub-meter accuracy, and combined with a low cost antenna, could be a good choice for some applications.